This invention relates to the field of sputter coating, and more particularly to the field of improving the quality of a thin film coated by sputtering techniques.
This application is related to the application entitled "Improved Cathode and Target Design for a Sputtering Apparatus," and applications entitled "Method and Apparatus for Handling and Processing Wafer-Like Articles," all filed on even date herewith.
Sputtering is an important technique for applying thin films to substrate materials, such as wafers utilized in manufacturing microelectronic components. The process can best be envisioned as a series of steps, occurring in a low-pressure chamber into which a gas, typically argon, has been introduced. A negative potential is applied to a cathode structure, inducing an electric field, and electrons collide with argon atoms, creating ions and exciting a glow discharge. Accelerated by the cathode's negative potential, the ions travel parallel to the electric field lines and impact a target, composed of the coating material, carried in the cathode structure. The kinetic energy of these ions is sufficiently high to dislodge some target surface atoms, which then condense on the substrate to form the film. Also, ion bombardment causes the emission of secondary electrons from the target, and those electrons ionize more argon atoms, and so on, in an "avalanche" effect.
One method used in the sputtering art has been the employment of magnetic fields to enhance plasma density, which in turn enhances the ion bombardment of the target. In such apparatus, referred to as magnetron sputtering devices, magnetic means are disposed to induce a relatively strong field in the vicinity of the target face, with the magnetic field lines oriented generally perpendicular to those of the electric field. Electrons emitted from the target face are influenced by the magnetic field so that their path of motion becomes curved, and in effect, the magnetic field traps the electron in the vicinity of the target. The effect of this action is to promote electron-argon collisions close to the target face, maximizing the ion flux bombarding the target. Typical of magnetron sputtering apparatus is the device disclosed in U.S. Pat. No. 4,472,259, assigned to the assignee of the present invention.
Sputtering is employed as discussed above (conventional sputter coating), or in one of three variant processes. If it is desired to remove material from a substrate, rather than deposit a film, the apparatus can be arranged so that the substrate becomes the target of the ion bombardment. Usually the object is to achieve the removal of material, for example, to prepare a silicon wafer for the deposition of interconnection and component material in other process steps. Additionally, it is possible to affect the substrate during etching by employing reactive sputter etching, in which a reactive gas, such as oxygen or chlorine, is added to the argon in the sputtering chamber. Radicals of the reactive gas react with the substrate to form volatile compounds, so that film removal proceeds by a combination of sputter etching and chemical actions.
Also, if ion bombardment of the substrate during formation of the film is advantageous, that result can be achieved through bias sputtering, in which a negative bias, typically of less magnitude than that applied to the cathode, is impressed upon the substrate. Under the influence of this bias, a "secondary" ion flux (as distinguished from the "primary" ion flux that bombards the target) impinges upon the substrate simultaneously with the coating material atoms. As reported by Chapman in "Glow Discharge Processes", pp. 231-32 (1980), bias sputtering can be particularly useful when attempting to coat three-dimensional surface configurations, typically microscopic steps and holes formed in the substrate surface, generally having depth and width dimensions under one thousandth of an inch. Such surfaces are typically found on microelectronic component wafers, and have long posed a problem for effective application of films, especially the uniform coverage of perpendicular walls.
A measure of the effectiveness of a given system in coating such surfaces is referred to as "step coverage," defined as the ratio of nominal film thickness to the minimum thickness, typically found at either the top or bottom corners of a vertical step. It will be appreciated that a low value of step coverage leads to early failure of electronic devices, as the increased resistance at such points causes material migration or heat buildup that can lead directly to circuit failure. Bias sputtering often is employed in such situations, but as Chapman notes, the problem remains acute.
Users of devices produced by sputtering technology, however, are now demanding products that stretch and exceed the capabilities of the present sputtering art. For example, typical wafer geometries now in the production stage call for surface steps three microns wide by one micron deep. Newer designs seek to pack more devices on a single chip, and require coating surface steps only one micron wide by one micron deep, and designs in the very near future will specify half that width.
When the shape of the step approaches a square configuration, the sides of the step begin to act as a mask, effectively blocking coating material atoms from reaching the bottom corners of the step. At the very least, this tendency produces reduced step coverage, and in the extreme can lead to the formatiom of "tunnels" or "mouse holes" in the coating, in which material applied to the sides of a step grows outward to make contact with material applied to the opposite side, leaving the interior almost completely devoid of coating. This effect can be very detrimental to performance of the final device, and would be most undesirable under proposed coating regimes in which planar coatings (all steps completely filled, and a further thickness of film atop the entire wafer, producing a substantially flat final surface) rather than conventional conformal coatings are required.
A complicating factor in bias sputtering is the presence of variations in the "secondary" ion flux, resulting from the effects of the magnetic field used to enhance bombardment of the sputtering target. As mentioned, one observed effect of bias sputtering is an improvement in step coverage, possibly through increasing the mobility of the surface material during film formation. It will be apparent that non-uniformities in flux across the surface of the substrate will produce detrimental effects. If it is important, for example, to achieve good step coverage, then a non-uniform "secondary" flux will result in a product having areas of poor coating quality, observable, for example as variations in step coverage in different areas of the substrate. When coating silicon wafers for the semiconductor industry, this non-uniformity directly leads to high loss rates.
Thus, non-uniformity of the "secondary" ion flux in a bias sputtering system stands as an obstacle to the effective employment of this technique in meeting the needs of the sputtering industry's customers. It is to this problem that the present invention is addressed.